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R. B. Husar and W. R. Shu

Abstract

It has been shown by earlier studies on the formation of the Los Angeles smog aerosol, that a significant fraction of the particulate matter in the visible size range is made by secondary gas-particle conversion processes, and that it consists mainly of liquid matter. In this study the volatility was studied quantitatively by heating the smog aerosol up to 250°C and simultaneously measuring the scattering coefficient using a nephelometer and the change of size distribution by an optical counter. The thermal analyses revealed that 50–80% of the submicron aerosol mass is volatile at 220°C. Inspection of the shapes of thermo-nephelograms (b scat vs temperature) also revealed that photochemically formed aerosols have distinctly different thermo-nephelograms than those obtained on humid “hazy” days. Thus it is suggested that thermal analyses may at least in some instances provide the means of on-line monitoring the origin of the ambient aerosol.

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D. E. Patterson, R. B. Husar, W. E. Wilson, and L. F. Smith

Abstract

The daily distribution of sulfate concentration over the eastern United States during August 1977 is simulated by a Monte Carlo model using quantized emissions, positioned in accordance with the 1973 EPA SO2 emission inventory. Horizontal advection within a single well-mixed vertical layer is driven by observed surface winds, speeded by a factor of 2.5 and veered 20°. Direct simulation of regional diffusion is implemented by random perturbation of each quantum's trajectory over each 3 h time step, corresponding to K = 105m2 s−1. First order kinetics of SO2 to SO4 transformation and deposition of SO2 and SO4 are simulated by probabilistic choice among the chemical and physical pathways over each step. The simulated spatial distributions are compared on a daily basis to measured sulfate concentration from the Sulfate Regional Experiment (SURE) and midday visual range observations, indicating the primary role of airmass residence time over the eastern United States in producing regional pollution. The light extinction coefficient, b ext and SURE SO4 show remarkable spatial and temporal agreement throughout the month. The correlations of daily average SO4 concentration and b ext over the SURE region with the Monte Carlo simulation are 0.63 and 0.67, respectively. The correlation between daily average b ext and measured SO4 is 0.84.

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